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Anti-aliasing filter with less than unity gain

Charles Steinkuehler
Prodigy 140 points

Other Parts Discussed in Thread: PCM1808, TINA-TI

I'm looking to use the PCM1808 in a new design and I need a front-end anti-aliasing filter with less than unity gain.  There is an example design like this on the evaluation board for the 1808 (gain of 1/2), but I'd like to map the -10 dBv consumer reference level to -20 dBFS on the 1808, which requires a different gain (-7.2 dB if I've got my math right).

I've tried using the FilterPro software, but it seems to force me to have a gain of unity or higher.

I'd like to use the single-ended multiple-feedback topology TI used in their eval. board, but my "roll your own" filter books don't cover design of this architecture (or I'm not familiar enough with the theory to do the required transforms).  Any recommendations for a tool that can help design these filters?

  • 0
    TI__Genius 16270 points

    Charles,

    Have you looked at TINA-TI?

    http://focus.ti.com/docs/toolsw/folders/print/tina-ti.html

     

  • 0 in reply to Patrick Galvin
    Prodigy 140 points

    I have many different simulation tools, what I was looking for was design support for filters with less than unity gain.

    I found useful information from Analog Devices in AN-649 (documentation for their filter design tool).

  • 0
    TI__Intellectual 935 points

    Hello Charles,

    The demo board for the part in question is the "DEM-DAI1808", whose datasheet is available here (http://www.ti.com/litv/pdf/sleu078). In this datasheet we have access to the anti aliasing filter already in place for this part. As you mentioned, the only difference is that you want to change the attenuation from -6 dB to -7.2 dB.

    Since this AA filter is a type of mutliple feedback low-pass filter, we are able to generalize and say that the DC gain of the filter is given by the following:

     
    ; where K is the DC gain (or attenuation) of the filter and R1 and R4 are given in the following picture (thanks to Wikipedia):
     
     
    So, all we have to do is find the appropriate ratio R4/R1 and change the resistor values.
     
    I just changed R105 in the demoboard data sheet from 1.2k to 1.05k. I then simuated this in TINA to get the following:
    That configuration is the same as the demoboard's (2 op amps per channel) and you satisfy your attenuation condition.
    I don't think I can attach the TINA file here, so I will post a picture of the schematic instead.
    To answer your question, I don't know of any tools to create a filter with less than unity gain. However, using this information it is possible to use FilterPro and modify the schematic to reduce the gain.
    Let me know if you need to get your hands on an eval board.
    Should you have any other questions or if you have an application that TI can help you with, do not hesitate to contact me or any applications engineer here at TI.

    |  Richard Barthel

    |  Analog Applications Engineer

    |  Texas Instruments Incorporated

  • 0
    TI__Intellectual 935 points

    Charles,

    Did this answer your question? If so, please verify my post as the answer you were looking for.

    If not, please let me know how else I can help you out.

    Thanks.

     

    -Rich